While it is possible that Apple was considering offering the ‌iPhone‌ 5s in Black and Slate, before opting for Space Gray instead, DongleBookPro suggests that the color was likely used to help ...

And it's a good question. "Slate is a sophisticated, muted tone that sits between gray and blue," explains Emily Kantz, a NCIDQ-certified designer and color marketing manager at Sherwin Williams.